Long Term Evolution (LTE) technology, introduced in 3GPP Release 8, is the next major step in mobile radio communications. It will give a superior user experience and support even more demanding applications, such as interactive TV, user-generated videos, advanced games, and professional services. LTE uses OFDM (orthogonal frequency-division multiplexing) radio access technology together with advanced antenna technologies.
FIG. 1 illustrates the LTE system architecture, including an E-UTRAN radio access network and an Evolved Packet Core (EPC) network. The E-UTRAN consists of eNodeBs (not shown), which provide the radio interface toward the User Equipment (UE). The eNodeBs are interconnected with each other via the IP-based X2 interface and toward the EPC by the IP-based S1 interfaces. The EPC comprises the Serving Gateway (SGW), the Mobility Management Entity (MME), the Packet Data Network Gateway (PDN GW), and the Policy Charging Rules Function (PCRF). Unlike the 3G architecture, EPC does not reuse the Home Location Register (HLR). Rather, a Home Subscriber Server (HSS) replaces the subscriber register functionality of the HLR in the EPC. In addition, a Serving GPRS Support Node (SGSN) acts as a mobility server or mobility management node for user terminals accessing the EPC via legacy radio access technologies such as a GERAN (2G) or UTRAN (3G).
The SGW sits in the user plane where it forwards and routes packets to and from the eNodeB and the PDN GW. The SGW also serves as the local mobility anchor for inter-eNodeB handover and roaming between two 3GPP systems. For a UE in idle mode, the SGW is responsible for terminating the down-link data path and when down-link data is received, buffering the data and triggering a paging procedure by signalling the MME over the S11 interface.
The MME is the control-plane function for E-UTRAN access. The MME manages states (attach, detach, idle, RAN mobility), authentication, paging, mobility with 3GPP 2G/3G nodes (such as the SGSN), roaming and other bearer management functions. It is responsible for authentication and critical management for mobile devices as well as for tracking and paging procedures for UEs in idle mode. The MME authorizes bearer activation/deactivation including SGW and PDN gateway selection. Like the SGSN, the MME maintains a knowledge of the current location of a subscriber on the cell level, and acts as a mobility server or mobility management node. The MME is connected to the E-UTRAN by the S1-MME interface, and to the HSS by the S6a interface.
The PDN GW acts as the interface between the LTE network and the Packet Data Networks (PDNs), such as the Internet or SIP-based IP Multimedia Subsystem (IMS) networks (fixed and mobile). The PDN GW is the mobility anchor point for intra-3GPP access system mobility and for mobility between 3GPP access systems and non-3GPP access systems. The PDN GW performs IP policy and charging enforcement on packet flows to and from mobile devices. The PDN GW is responsible for IP address allocation, charging, deep packet inspection, lawful intercept, policy enforcement and other services.
In order to allow LTE subscribers to make use of 2G/3G services (e.g. in geographic locations where LTE is not available), seamless mobility between the different radio access technologies (RAT) should be available. A function known as Idle mode Signalling Reduction (ISR), specified in 3GPP TS 23.401, provides a mechanism to limit signalling during inter-radio access technology (inter-RAT) cell reselection (i.e. a handover between a 2G/3G RAT and a LTE RAT) when a UE is in Idle mode.
UEs that are currently participating in an active session (data or voice) are said to be in active mode. In active mode, the network knows the exact cell in which the UE is located. When the UE is not performing an active data transfer, it is said to be in idle mode. In idle mode, the network does not know the exact cell in which the UE is currently camping. Instead it knows the location of the cell only to the granularity of a group of cells (e.g. a Routing Area or Tracking Area). Therefore, in idle mode the network needs to page the UE in all the cells in this group in order to deliver UE-terminated data.
The ISR mechanism allows the UE to remain simultaneous registered in both an UTRAN/GERAN Routing Area (RA) and an E-UTRAN Tracking Area (TA) list. This allows the UE to make cell reselections between E-UTRAN and UTRAN/GERAN without a need to send any TA Update (TAU) or RA Update (RAU) requests, as long as it remains within the registered RA and TA list. For example, when a UE moves from an SGSN to an MME, the MME sends a Diameter Update Location Request message to the HSS indicating that ISR applies. Consequently, the HSS does not send a Cancel Location Request message to the SGSN. When the user moves back to the SGSN and ISR applies, the SGSN does not send a MAP Update GPRS Location message to the HSS (over the Gr interface). Maintaining 2G/3G bearers for a relatively short period will consume fewer network resources than would be consumed when performing frequent inter-RAT handovers. Such functionality is desirable as it is expected that, at least in the initial rollout phase, LTE access will be limited to relatively small “hot spots” and as such inter-RAT handovers will be frequent.
When ISR is activated this means the UE is registered with both MME and SGSN. Both the SGSN and the MME have a control connection with the Serving GW and are both registered at HSS. The UE stores Mobility Management (MM) parameters from the SGSN (e.g. P-TMSI and RA) and from the MME (e.g. GUTI and TA(s)) and session management (bearer) contexts that are common for E-UTRAN and GERAN/UTRAN accesses. In idle state the UE can reselect between E-UTRAN and GERAN/UTRAN (within the registered RA and TAs) without any need to perform TAU or RAU procedures with the network. In addition, the SGSN and the MME store each other's address when ISR is activated. However, ISR does create more complex paging procedures for UEs for which ISR is active, as there is then a need to page the UE on both the registered RA and all registered TAs. For example, when downlink data arrives that is destined for a UE for which ISR is activated (and the UE is in idle mode), the Serving GW initiates paging processes on both the SGSN and the MME. Similarly, when the HSS receives a request for the current location of a UE for which ISR is activated, it will not know which of the two possible accesses the UE is using, and will therefore need to request that both the SGSN and the MME page the UE. The UE will then only respond from the radio access network on which it is currently camped.
This simultaneous “double registration” of the UE with both the MME and the SGSN has been included within 3GPP Release as a means to support ISR. However, there may be additional situations in which a double registration may occur, even though ISR is not in use. For example, when a UE switches from a UTRAN/GERAN to an E-UTRAN the HSS may not be informed of the change in access, such that the HSS has registrations pointing at both a MME and a SGSN for that UE, even if ISR is not in use.
These double registrations can also create problems in scenarios in which an application server (AS) requests the location and/or status information of a UE. In these scenarios the HSS can not determine which of the two registrations is the most recent. In addition, if a UE which has a double registration changes between an LTE access and a 2G/3G access, and provided the change is between the same MME-SGSN pair, then the HSS will not be notified about this access change. As such, the information that the requesting AS receives from the HSS may not be accurate, and may therefore cause the AS to initiate a location or status information request which may be sent to the wrong mobility management node (i.e. MME/SGSN).
For example, in order to provide Location Services (LCS) in the case of a Mobile Terminated Location Request (MT-LR), a Gateway Mobile Location Centre (GMLC) issues a request towards the HSS via the Lh interface for the identity of the mobility management node at which the UE is currently registered. However, if a UE has a double registration with both an SGSN and an MME, then the HSS will respond with a list including the identities of both the SGSN and the MME. As such, the GMLC will not know to which of the two nodes it should send the request for the UE's location (even though the HSS, as an implementation option, can set one of the mobility management nodes as the “main entity”).
In addition, both SMS and CS telephony in 2G/3G require the Charging Data Records (CDRs) to include a network validated Cell ID. This Cell ID may be used for charging purposes but may also be necessary to meet regulatory requirements. However, for similar applications in an LTE system, such as SMS over IP and MMTel, if the UE is registered with both the MME and SGSN and in idle mode, then the Cell ID sent in the last update to the HSS may well not be the current Cell ID. As such, the Cell ID provided by the HSS to application servers supporting such services may well be inaccurate.
By way of further example, in some networks there are third party application services that make use of the Any Time Interrogation (ATI) feature of the Customised Applications for Mobile network Enhanced Logic (CAMEL) standard. This ATI process enables a GSM Service Control Function (gsmSCF) to interrogate a HLR, or an IP Multimedia Service Switching Function (IM-SSF) to interrogate a HSS (on behalf of a gsmSCF), for location and or subscriber information. As such, these applications are typically interested in the location (e.g. Cell ID) and the status (i.e. idle or active) of the UE. However, if an AS such as an IM-SSF requests information relating to a UE that has a double registration, then the HSS may respond with information that is not accurate. For example, the HSS may indicate that a UE is visiting an MME, as this is configured within the HSS as the main entity, when the UE is in fact located at an SGSN.
Furthermore, for voice services in LTE networks there is currently no mechanism that allows the network to know whether a UE is in an access (typically LTE) where MMTel based voice services can be used or whether a voice call should be directed to a circuit-switched (CS) access (typically 2G/GSM). According to current network functionality, for a Mobile Terminated (MT) call, the Terminating Access Domain Selection (T-ADS) function of the Service Consistency and Continuity (SCC) AS would try to set up an MMTel call over the PS access (typically LTE access). Then, if the UE is in a 2G access network, the SIP INVITE will have to be routed all the way to the UE over the 2G packet-switched (PS) access. As the UE will not have support for Voice over IMS in the 2G access network, it will then have to respond rejecting the INVITE. Upon receiving the rejection the SCC AS will then have to contact the MSC in order to set up a CS call. This leads to a long call setup time when a UE is visiting in a 2G access of a multi-access network (i.e. GSM and/or UMTS radio access networks together with an LTE radio access network).